To prevent contamination of wafers and generation of a natural oxide film, the process of producing semiconductors is generally carried out in a clean room. Depending on the ambient air quality, etc., some of the chemical components in the ambient air are removed by an activated carbon filter. The air is then introduced into the clean room by an air-conditioning system at a constant temperature, 23-25.degree. C. for example, and a constant relative humidity, 40-50% for example.
However, advancements in the integration of semiconductors require that the manufacturing process be carried out in an atmosphere cleaner than the air provided by the prior art filter/air-conditioner system. That is, the chemical components and moisture content left in the clean room air by the system described above still constitute a contamination source hindering the production of semiconductors.
In addition to nitrogen, oxygen and noble gases (Ar, Ne, Kr, etc. belonging to group VIII of the periodic table), the prior art system leaves in the clean air diverse chemical components such as hydrocarbons, organic halogens, acidic gases, basic gases, aldehydes, nitrogen oxides, etc., and these diverse chemical components may contaminate semiconductor wafers independently or in the presence of moisture, even if their concentration is extremely small. The degree of humidity secured as described above is provided to alleviate dryness in the mouths of workers, etc., but when air having this degree of humidity is present, a natural oxide film easily forms on wafers in contact with such air. For example, when the dew point is around -90.degree. C. (moisture content 0.1 ppm), natural oxide film is scarcely formed even after a lapse of some 100 hours, but when the dew point is around 50.degree. C. (moisture content 1.2 vol %), a natural oxide film will form in a few minutes.
Therefore, in order to prevent this kind of wafer contamination and the generation of a natural oxide film, super clean air must be used as the air with which the wafer comes in contact. This super clean air must have chemical components and moisture content removed therefrom to the same degree as super high quality gases such as the nitrogen gas used in the semiconductor manufacturing process. Specifically, in order to prevent wafer contamination, the chemical components in the air must be removed such that the remaining chemical components do not exceed a part per billion (ppb) level, equivalent to the level of chemical components contained in super high quality gases. Also, in order to prevent the generation of a natural oxide film on the wafer, the dew point of the air must be lowered to -40.degree. C. to -120.degree. C. (preferably -100.degree. C. or lower)
The requirement for super clean air causes the production costs to be extremely high as compared to clean room air adjusted and controlled by an air-conditioning system as described above because it is difficult to provide super clean air as the atmosphere for a large volume such an entire clean room. To reduce costs, a region is formed in the clean room where wafers come in contact with air (for example, the wafer transport region between processes) whose volume can be greatly reduced within a range which does not hinder semiconductor production, the region being originally designed to be of a small volume from the viewpoint of functions and applications.
To minimize the volume of super clean air required and at the same prevent as much as possible the contamination of wafers and the formation of natural oxide films during wafer transport, a tunnel type wafer transport system has been proposed. In this system the wafer transport region is formed in a tunnel with the minimum required volume for transporting wafers, and super clean air is supplied to the tunnel rather than to the entire clean room. It has also been proposed to supply super clean air to places other than the tunnel (for example, the inner space of a storage apparatus) where contact of the wafer with air is likely to permit contamination or formation of a natural oxide film. To prevent wafer contamination, etc. it has also been proposed to supply nitrogen gas to the tunnel, etc. in place of super clean air, but the use of nitrogen is not practical from the viewpoint of cost and safety to humans.
Thus, while it is possible to effectively prevent wafer contamination and the formation of natural oxide films by supplying super clean air in the form of nitrogen to the wafer transport tunnel, etc., it is preferable that the super clean air to be produced from atmospheric air so as to reduce costs, improve safety, etc. However, the chemical components in the atmosphere are diverse as described above, and because the types of chemical components contained in material air differ widely depending on where the air is collected from the atmosphere, it is extremely difficult to remove all the chemical components in the material air such that the remaining chemical components are so scarce as to stand at no more than about one ppb. Furthermore, no method has been proposed for producing super clean air efficiently and in large quantities by using atmosphere as the material from which the super clean air is derived. This has hindered the practical applications of the tunnel method to the wafer transport system, etc. described above.